It is well known that an internal combustion engine tends to be fired in an improper manner when it is operated at a relatively low load such as when it is decelerated from high speed operation, when an engine brake is applied thereto or when it is idling. Under such condition, the engine tends to cause uncomfortable noise. Especially, a rotary engine has such tendency to a considerable degree and, therefore, irregularly repeats a fire and a misfire with the result that variation in torque and uncomfortable noise occur.
Of late, an ignition system has been proposed that is adapted to control an ignition circuit or circuits so that the ignition can be repetitively effective and ineffective in a forced manner in time with the tendency of fire and misfire. In the prior art, an ignition system of such a type comprises two sets of ignition circuits each including an ignition coil and a contact breaker connected to the primary side of the ignition coil, with the contact breaker of one of the ignition circuits having the interruption frequency of one half that of the other contact breaker. In the normal operation, the ignition having the higher interruption frequency is operated so that the ignition can be normally effected, but when the engine tends to be fired in an improper manner as previously described, the engine is switched to the other ignition circuit having the lower interruption frequency so that the engine is fired at a frequency one half that of the normal operation. However, such ignition system necessitates a reduction mechanism which is required to reduce the frequency of operation of the contact breaker of the one ignition circuit, which causes the construction of the apparatus to be complicated.